This invention relates to a differential amplifier having a current mirror circuit as a load for a differential amplifying circuit and suitably used with low power source voltage.
In FIG. 1, NPN transistors Q1 and Q2 constitute a differential amplifying circuit. The emitters of the transistors Q1 and Q2 are connected together. A constant current source Io is connected between the emitter junction of the transistors Q1 and Q2 and ground. A current mirror circuit CM is provided between the transistors Q1, Q2 and a power source voltage V.sub.CC. The current mirror circuit CM comprises PNP transistors Q3, Q4 and resistors R1, R2. The base and collector of the transistor Q3 are connected together. A reference or primary current flows through the transistor Q3. The base of the transistor Q3 is connected to the base of the transistor Q4. A secondary current flows through the transistor Q4. The resistor R1 is connected between the emitter of the transistor Q3 and the power source voltage V.sub.CC. The resistor R2 is connected between the emitter of the transistor Q4 and the power source voltage V.sub.CC. Input terminals 1 and 2 are connected to the bases of the transistors Q1 and Q2, respectively. The output terminal 3 is connected to the collector of the transistor Q1.
The differential amplifier in FIG. 2 is different from that in FIG. 1 only in the structure of the current mirror circuit. The current mirror circuit in FIG. 2 is shown by a reference character CM'. The circuit CM' comprises the PNP transistors Q3, Q4 and PNP transistors Q5, Q6. The transistor Q5 is connected between the emitter of the transistor Q4 and the power source voltage V.sub.CC. The emitter and base of the transistor Q5 are connected together. The transistor Q6 is connected between the emitter of the transistor Q3 and the power source voltage V.sub.CC. The base of the transistor Q6 is connected to the base of the transistor Q5.
In the current mirror circuits CM, CM', assume that the primary current and secondary current are equal. In this case, when a signal potential V1 at the input terminal 1 is higher than the signal potential V2 at the input terminal 2, a current flows through the output terminal 3 into the collector of the transistor Q1 as a part of the collector current, together with the current flowing from the transistor Q4 into the collector of the transistor Q1, so that the collector current of the transistor Q1 is larger than that of the transistor Q2. While, when the signal potential V1 is lower than the signal potential V2, a part of the collector current flowing from the transistor Q4 into the collector of the transistor Q1 flows out through the output terminal 3.
A voltage drop of the current mirror circuit CM in the differential circuit of FIG. 1 is relatively small. Therefore, the amplifier will operate with a power source of a relatively low voltage.
However, the base currents of the transistors 03, Q4 of the current mirror circuit CM may not be ignored when amplification factors of the transistors 03, Q4 are small. When the amplification factors of the transistors Q3, Q4 are small, the operation characteristic of the current mirror circuit CM is low. This causes a large offset of an output current through the output terminal 3. Also, with the amplifier of FIG. 1, the so called Early effect (i.e., in which the collector emitter voltage varies, according to the variation of the power source voltage, to vary the collector current) at the transistor Q4 varies the offset of the output current. Therefore, the amplifier of FIG. 1 is unsuitable for use with a wide range of the power source voltage used. The resistors R1, R2 are provided in the current mirror circuit CM in order to reduce the Early effect. This measurement, however, cannot significantly reduce the Early effect.
With the differential amplifier of FIG. 2, the transistors Q5, Q6 reduce the affection of the base currents of the transistors Q3, Q4 on the offset of the output current. This enhances the operation characteristic of the current mirror circuit CM'. However, this measurement still does not satisfactorily reduce the Early effect. Although small, the offset of the output current will still occur. Also, with the amplifier of FIG. 2, the diode connected transistor Q5 is connected between the emitter of the transistor Q5 and the power source voltage V.sub.CC. The voltage drop at the transistor Q5 is relatively large. Therefore, the FIG. 2 amplifier is not suitable for use with a relatively low power source voltage, e.g., 0.9 V.